This invention relates to an image encoding apparatus, an image transmission apparatus and an image filing apparatus etc.
In order to increase the efficiency of image transmission or image storage, many image encoding methods or image compression methods are known.
These methods are generally divided into the following two categories:
(1) Information preserving type encoding, and
(2) Information non-preserving type encoding
Information preserving type encoding is mainly used for encoding an image in which the amount of data is relatively small, such as black and white binary images.
A typical information preserving type encoding is MH coding or MR coding, used to encode a black and white binary image for a facsimile transmission.
Recently, a dynamic arithmetic encoding is technique has been developed, which encodes an image while dynamically predicting a pixel.
On the other hand, the alternative information non-preserving type encoding is mainly used for encoding an image in which the amount of data is relatively large, such as a full colour image.
For example, an encoding method that converts an 8-bit R,G,B signal into a luminance signal and chrominance signal, performs an orthogonal transformation (a discrete cosine transformation) on the converted signal, quantizes the transformed signal and Huffman codes the quantized signal has been proposed.
This encoding method reduces the amount of image data by deleting high spacial frequency images and saving low spacial frequency images. Accordingly, this encoding method is lossy and non-reversible. The encoding technique is efficient but there is a trade-off in terms of image deterioration.
The information non-preserving-encoding technique has an advantage in that it can efficiently encode a colour halftone image, with only a small deterioration in image quality. However, it has a disadvantage in that significant image deterioration occurs when a colour image having high spacial frequency, such as including characters or line drawings, is encoded.
In view of these circumstances, the assignee of the present application has proposed in U.S. Ser. No. 577312 (CF07021 US. EP) an encoding method which converts a colour image, e.g. characters or line drawings, into a binary colour image and then encodes the binary colour image using information preserving-type encoding.
In this proposed encoding method, a colour image having characters or line drawings, can be efficiently encoded, without a deterioration of the image.
Not only characters or line drawings but also a binary colour image obtained by pseudo halftone processing, e.g. dither method or error diffusion method, can be efficiently encoded without deterioration of an image by using the information preserving-type encoding.
When a binary colour image is encoded by information preserving-type encoding, the above proposed method separates a binary colour image into 3 planes and encodes each of the 3 planes, invididually. Namely, 3 planes of binary colour image are encoded plane sequentially (plane sequential encoding).
However, if a printer of the receiver side is not compatible with plane sequential encoding, for example, when the printer is a dot sequential printer and not a plane sequential printer, plane sequential encoding cannot be used, unless the receiver side is capable of converting plane sequential data into dot sequential data.
On the other hand, in cases where a binary colour image is encoded dot sequentially and not plane sequentially, if the printer of the receiver side is a plane sequential printer, the dot sequential encoding cannot be used unless the receiver side is capable of converting dot sequential data into plane sequential data.
If the receiver side can accept both plane sequential encoding and dot sequential encoding, it is desirable to encode the binary colour image by a more efficient encoding technique, because the efficiencies of encoding are different from each other, even if an identical image is encoded.
When a prediction coding technique, such as dynamic arithmetic encoding, is used as the information preserving-type encoding, a binary colour image representing a character or line drawings has a strong correlation between planes, so the efficiency of encoding can be increased by predicting an image to be encoded by referring to other planes.
However, a binary colour image obtained by a pseudo halftone process, such as the error diffusion method etc. has a weak correlation between planes, so the efficiency of encoding is likely to decrease when the prediction is performed by referring to other planes.
Whether other planes are referred or not must be decided in consideration of not only the content of an image to be encoded but also the decoding function of the receiver side.
Moreover, when a single plane of an image has both line drawings and half tone image, it is difficult to decide which encoding technique, namely referring other planes or not, is the more efficient. Accordingly, if the decision whether other planes are to be referred or not can be decided without the decision of an operator, binary colour image can be encoded with the most suitable encoding method.